1. Field of the Invention
The present invention relates generally to an apparatus for measuring the differential mode delay of a multimode optical fiber and, more particularly, to an apparatus for measuring the differential mode delay of a multimode optical fiber using a tunable laser source and optical frequency domain reflectometry.
2. Description of the Related Art
Recently, interest in realizing gigabit Ethernet by using an 850 nm Vertical Cavity Surface Emitting Laser (VCSEL) is increasing. In order to achieve such a gigabit Ethernet, research into a MultiMode optical Fiber (MMF) is actively being conducted. In the research of the MMF, the analysis of Differential Mode Delay (DMD), which is one of the important characteristic evaluation factors, is essential in order to evaluate MMF performance, so that a lot of research into MMF has recently been conducted.
FIG. 6 shows a conventional apparatus for measuring the DMD of an MMF. The conventional apparatus includes a pulse laser 61, a high-speed light detector 62, and a sampling oscilloscope 63. The pulse laser 61 applies a short laser pulse, which oscillates only for a specific time, to the MMF. The short laser pulse is resolved due to mode separation and difference in traveling velocity between modes while passing through the MMF. The intensities of resolved pulses based on mode are converted into electronic signals by the high-speed light detector 62. The sampling oscilloscope 63 measures the time delay between the pulses, which have been converted into electronic signals, in a time domain.
The conventional apparatus for measuring differential delay in a time domain requires the short laser pulse and the high-speed sampling oscilloscope and, therefore, increases the cost of the optical fiber because the prices for realizing the short laser pulse and the high-speed sampling oscilloscope are high and, therefore, the cost of an optical fiber test increases. It is further difficult to provide the above-described high-priced optical fiber test equipment to every optical fiber production line for the tests of optical fiber, so that a lot of time for testing of optical fiber is required and, therefore, a barrier exists to the mass production and sale of optical fiber. Furthermore, in conventional time delay measurement, modes are separated when an optical fiber has a length more than 500 m, so that a mode delay characteristic can be analyzed only for an optical fiber longer than 1 km, but cannot be analyzed for an optical fiber having a short length. Accordingly, in order to determine the mode delay characteristic of the MMF having a length less than 1 km, the development of a measurement apparatus that can analyze an optical fiber having a short length is required.